1,3-Dichloropropene has isomers i.e. cis-1,3-dichloropropene (hereinafter referred to as a cis-isomer) and trans-1,3-dichloropropene (hereinafter referred to as a trans-isomer) and is known as a compound useful in the medical and agricultural fields.
Heretofore, as a method for separating such a cis-isomer and a trans-isomer, e.g. JP-B-53-15484 discloses a process wherein a reaction product obtainable by a reaction of propene with chlorine, is distilled to obtain 3-chloropropene, and then, from the bottom after the distillation, the cis-isomer and the trans-isomer of 1,3-dichloropropene are separated. Namely, in the above bottom, residual substances such as 1,3-dichloropropene and C6 olefins, are contained as by-products of the reaction. Firstly, chlorine or bromine is reacted to the bottom to selectively halogenate the C6 olefins, and then, this bottom is distilled to remove a light component having a boiling point lower than 1,3-dichloropropene. Thereafter, the bottom is further distilled to remove the halogenated C6 olefins as a high boiling component and to fractionate 1,3-dichloropropene as a low boiling component. And, such 1,3-dichloropropene is further distilled to separate the trans-isomer as a high boiling component and the cis-isomer as a low boiling component.
In the above process, a lower boiling component and a higher boiling point component than the cis-isomer and the trans-isomer, are sequentially removed by distillation, and thereafter, the cis-isomer and the trans-isomer are fractionated, and this fraction is distilled to separate the cis-isomer and the trans-isomer. In such a case, the trans-isomer has a boiling point higher than the cis-isomer, and accordingly, the trans-isomer is separated as a high boiling component. Accordingly, the trans-isomer as the high boiling component, has had a problem that distillation residues are likely to be included, or the purity is low and it contains coloring components substantially.
Further, in order for the trans-isomer to be used as a material for e.g. an intermediate for pharmaceuticals, an intermediate for agricultural chemicals or an intermediate for functional materials, it is required to have a Hazen color number (APHA) of at most 20. Whereas, the trans-isomer obtainable by the conventional process has had a problem that it is substantially densely colored with its APHA being about 500.
Namely, it is an object of the present invention to solve the above problem and to effectively obtain a less colored trans-isomer of high purity.